As we grow older, our skin—the human skin—ages; elasticity is lost and wrinkles appear. As compared with skin areas protected from light, other skin areas that tend to be constantly exposed to sunlight, especially to ultraviolet irradiation (UV), i.e., the face, neck, and shoulders, produce significant wrinkles, sagging skin, age spots, and freckles as a result of chronic exposure to UV rays. UV rays also act to lower elasticity of the skin, increase darkening or yellowing of the skin, and reduce the moisture content of the keratinous layer. The skin aging phenomenon uniquely occurring in UV-exposed skin areas is called photoaging. In order to elucidate the photoaging mechanism of the skin and to evaluate cosmetics, pharmaceuticals, and similar products which are thought to be useful for preventing or mitigating unfavorable outcomes of photoaging; i.e., wrinkles, age spots, and freckles, a photoaged skin model would be useful if it reflects conditions of the human photoaged skin more faithfully.
For creating a photoaged skin model or an animal model, chronic exposure to UV-rays has usually been performed. Hitherto, the following exemplary methods have been known: a method in which the back skin of a hairless mouse or the paw pad skin of a rat is continuously irradiated with UV rays every day for several weeks ((1) Bissett D L, Hannon D P, and Orr T V, Photochem Photobiol. 1987; 46(3): 367-78, (2) Imayama S., Nakamura K., Takeuchi M., Hori Y., Takema Y., Sakaino Y., and Imokawa G., J. Dermatol Sci. 1994 7(1): 32-8)) and a method in which a three-dimensional model skin is irradiated in vitro with a single dose of UV light ((3) Nelson D., Gay RJ, Photochem Photobiol. 1993; 57(5): 830-7).
In relation to a model established on the basis of the human skin, there has been known a method in which a nude mouse to which human skin is transplanted is irradiated with a single dose of UV light ((4) Del Bino S., Vioux C., Rossio-Pasquier P., Jomard A., Demarchez M., Asselineau D., and Bernerd F., Br J Dermatol. 2004; 150(4): 658-67). However, in this model system, daily, continuous irradiation for several weeks is not envisaged. Accordingly, although this prior art method makes use of human-derived skin, a sufficient photoaging condition cannot be induced in the transplanted human skin, because UV irradiation is performed for only a short period of time. Thus, the model requires improvements before being recognized as a satisfactory human photoaged skin model. Presently, there has never been obtained a satisfactory human skin model or an animal model that better mimics human photoaged skin.